1. Field of the Invention
The present invention relates generally to implantable cardiac stimulation devices. The present invention more particularly relates to methods and systems for performing pacing interval optimization.
2. Background Art
Implantable cardiac stimulation devices are well known in the art. Such devices may include, for example, implantable cardiac pacemakers and defibrillators either alone or combined in a common enclosure. The devices are generally implanted in a pectoral region of the chest beneath the skin of a patient within what is known as a subcutaneous pocket. The implantable devices generally function in association with one or more electrode carrying leads which are implanted within the heart. The electrodes are positioned within the heart for making electrical contact with the muscle tissue of their respective heart chamber. Conductors within the leads couple the electrodes to the device to enable the device to deliver the desired electrical therapy.
Traditionally, therapy delivery has been limited to the right side of the heart. However, new lead structures and methods have been produced and practiced for also delivering cardiac rhythm management therapy to the left heart. These lead structures and methods provide electrode electrical contact with the left atrium and left ventricle of the heart by lead implantation within the coronary sinus of the heart. As is well known, the coronary sinus passes closely adjacent the left atrium, extends into the great vein adjacent the left ventricle, and then continues adjacent the left ventricle towards the apex of the heart.
It has been demonstrated that electrodes placed in the coronary sinus and great vein may be used for left atrial pacing, left ventricular pacing, and cardioversion and defibrillation. These advancements enable implantable cardiac stimulation devices to address the needs of the wide patient population, from those that would benefit from right heart side pacing alone, to those that would benefit from left heart side pacing in conjunction with right heart side pacing (bi-chamber pacing), to those that would benefit from left heart side pacing alone.
Pacing interval optimization has been conventionally performed while a patient in a resting state. For example, a first test pacing interval (i.e., delay) is delivered for a period of three minutes and a measure of cardiac performance is obtained at the end of the period. The process is then repeated for each of several other test intervals (i.e., delays), and a best delay is derived from the ensemble of measurements. Algorithms are then used to hypothesize as to what pacing intervals should be used when the patient's heart rate is elevated. However, because the data used in such algorithms is obtained while the patient is at rest, the accuracy of such algorithms are limited. In other words, a calculated optimum pacing delay for an elevated heart rate, which was calculated based on measures obtained when the patient was at rest, may significantly differ from the actual optimum pacing delay at the elevated heart rate. Accordingly, it would be beneficial to provide improved methods and systems for determining optimum pacing intervals at elevated heart rates, and more generally, at different physiologic states.
A patient may experience an elevated heart rate for any number of reasons, such as physical exertion due to walking, jogging, or climbing stairs, emotional stress, or as part of the normal physiologic response to pain or infection. A possible reason that pacing interval optimization has been conventionally performed while a patient is in a resting state is because motion artifacts, e.g., resulting from patient motion, may corrupt measures of cardiac performance obtained while the patient's heart rate is elevated. Accordingly, it would be beneficial if the effects of motion artifacts could be minimized.
Conventionally, pacing interval optimization is performed in a clinic. Common current techniques include the use of echocardiography to maximize aortic flow or optimizing diastolic filling. However, such convention approaches are cumbersome, time-consuming, and expensive. Additionally, such approaches are highly sensitive to user skill, and therefore can be of marginal accuracy and precision. Additionally, conventional approaches are typically performed with the patient at rest in a supine or recumbent position, which is not representative of the preload, afterload, and autonomic status that is most common during daily activities. Finally, conventional pacing optimization using external measurement systems are necessarily limited to a single point in time, such as immediately following device implant or during periodic follow-up visits. The calculated optimum intervals in such settings thus do not track the changes in the true, underlying optima that can be expected with changes over time due to fluid status, posture, level of autonomic tone, and the like.
One specific approach, to optimization that is known in the art is often referred to as “gradient ascent”. In one version of this approach, the hemodynamic response associated with a current pacing interval is compared to that induced by a test pacing interval. If the response improves, the current interval is replaced by the test interval. Otherwise, a different test interval is tried. This approach avoids retaining numerous measurements associated with a variety of test values which were obtained over an extended period of time. In theory, the device continuously evolves toward the best interval, and if the cardiovascular system is stable it should converge to the best value. However, the gradient ascent technique is only workable if the system is low in noise. When noise, measurement uncertainty, or variability in the underlying system is expected, the gradient ascent technique will typically fail to converge to a true optimum.
It would be beneficial to provide methods and systems for pacing interval optimization that overcomes some, and preferably all, of the above limitations.